I was just awarded the woot (enthusiast) badge on Stack Overflow, but I definitely didn't visit the site every day for 30 days. Here is my theory on how this happened.

When you open a new tab in Safari 4 it displays a new feature called 'Top Sites' which shows you previews for the front pages of your frequently visited sites. Stack Overflow shows up in my 'Top Sites', so I suspect that when Safari generates an up to date preview the 'visited today' count on Stack Overflow gets incremented.

If this is the problem then I doubt it's something that you can easily fix, but I thought I'd point it out.

Wait, how do you fix something like this? I'm just curious...
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Maxim ZaslavskyNov 11 '09 at 1:24

My guess is a combination of browser agent detection and activity heuristics (kind of like a CAPTCHA) to determine if a human actually did anything. It may be why people who merely read now and then are missing it.
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SynetechFeb 16 '10 at 16:35

@Synetech: Wouldn't the user agent be identical to when a user is browsing? Also, how could CAPTCHAs be used in this situation, as the Top Sites feature only loads the home page? :)
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Maxim ZaslavskyJul 13 '10 at 23:56

This would affect Opera for most users too; at least stackheads with the page set in Opera's speed-dial.

Opera uses the speed-dial (customizable grid of favorite pages miniaturized) as default for new tabs; not 100% sure what reloading policy that governs by default, but it would be a sensationally crappy feature if it didn't at least do it once a day ;-)

That being said, I'm an avid Opera and SO-user...wait...no StackOverflow in speed-dial...will keep sentence in answer, and mayhap post as a silver-medalist in a month.

Opera takes a snapshot when you set the speed dial. Not every time it's loaded. If you want to be certain, just hit reload on the speed dial page.
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randomDec 4 '09 at 1:12

Wasn't 100% sure of the mechanism, and haven't got the time to indulge in this delicious little nugget right now, but a quick glance at my current speed dial shows updated thumbnails (I can barely make out a 5 and a December on Coding Horror, and I now I haven't put it anywhere near my cache for quite a few days)....It would be a terrible feature (it already suffers from complete uselessness (at least for my surfing habits)) if it's default was never to update the thumbnails....
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Morten BergfallDec 6 '09 at 22:21

Safari 4 beta would show a slightly different User Agent for Top Sites than for actual usage. This is no longer the case in the current 4.0.2 release of Safari (at least not on a Mac; not sure when it changed).